The goal of the proposed experiments is to understand the mechanisms and rules which govern modification of synaptic strength in the cerebral cortex throughout life. Three hypotheses about experience-dependent plasticity will be tested: 1) that experience-dependent synaptic plasticity is partly subcortical early in life and cortical thereafter, 2) that cortical plasticity in older animals is mainly due to the changes in intracortical layer II/III pathways, 3) that experience-dependent plasticity, and refinement of somatotopic order, requires post-synaptic activation of NMDA receptors in the cerebral cortex. Three sets of experiments are proposed to test these postulates: 1) studies to test whether the changes in vibrissa representation in the cortex caused by uni-vibrissa rearing (trimming or pulling out all but a single vibrissa from birth) depends on cortical or subcortical plasticity, 2) experiments to test if and when plasticity ends in layers II/III of the cortex in older animals, and 3) studies to test whether glutaminergic cortical EPSPs are necessary for experience-dependent refinement of cortical somatotopy. These experiments will test the degree to which particular intracortical and thalamocortical pathways undergo experience-dependent plasticity at different ages. They will narrow down the receptor mechanisms involved by testing whether glutamate receptors, and in particular NMDA receptors, are necessary for experience-dependent plasticity and establishment of somatotopic order in the neocortex. By identifying the mechanisms and pathways involved in plasticity, several objectives may be achieved: the capacity for modification could be extended in cases of impaired development; insight could be gained into memory and learning deficits in adults; new approaches could be envisioned for restoring cortical function after brain damage. The detailed nature of these experiments will provide considerable information for studies on the mechanisms of plasticity in the future.